It is often necessary to expand a tube radially to form a leak-proof joint between the exterior of the tube and a surrounding tube sheet. Either roller swaging or hydraulic swaging can be used to produce tube expansion. Roller swaging employs a mechanical implement inserted in the tube and pressed against the tube surface, forcing the tube wall radially outwardly. The roller is repeatedly passed over the interior tube surface until the desired expansion has been produced.
Hydraulic swaging, which is generally superior, particularly for high pressure applications in small diameter tubes, employs a mandrel which is inserted in the tube to seal two ends of an elongated annular volume. Hydraulic fluid under pressure, which may be as high as 30,000 psi or more, is then introduced to the annular volume between the mandrel and the tube, forcing the tube to expand.
The use of swaging to expand tubes radially is of great importance in the construction of heat exchangers, particularly those heat exchangers intended for use in power plants. Hydraulic swaging, using pressures among the highest attainable, is found to be the most effective for this demanding application.
A problem that arises in this type of operation is that of anchoring the tube within the tube sheet in preparation for the high pressure swaging operation. One solution is to pre-expand a portion of the tube against the tube sheet prior to the insertion of the mandrel. It has been found, however, that the inner mandrel seal often engages an unexpanded portion of the tube while the outer mandrel seal engages the pre-expanded portion. When fluid pressure is supplied, the axial forces acting on the mandrel are unbalanced due to the unequal diameters of the tube at the location of the two seals, and consequently, the mandrel tends to move axially within the tube.
An objective of the present invention is to provide a new and improved method for expanding a tube radially. A further objective is to provide such a method that results in the generation of balanced axial forces in the mandrel.